资源环境科技发展态势分析平台

Metal‐bound organic carbon (OC) is vital for the persistence of soil organic carbon (SOC). However, its relative importance in different land types (e.g., upland vs. wetland) remains unclear. Here by analyzing metal‐bound OC by the classic citrate‐bicarbonate‐dithionite method (OC_CBD) from 62 soils from contrasting environments, we find that OC_CBD constitutes an equally important proportion of SOC (OC_CBD%) in wetlands as in uplands. Land‐use type and calcium (Ca) contents are the best predictors of OC_CBD%, likely owing to the contribution of OC directly associated with sulfate‐extractable Ca (OC_Na2SO4) and Ca’s bridging and aggregating effect on OC_CBD. By further separating OC_Na2SO4, we find that OC_Na2SO4 accounts for ∼24% of OC_CBD in Tibetan‐Mongolian grasslands and its relative contribution to OC_CBD shift in an S‐shaped mode along the pH gradient (6–9). These findings highlight Ca’s important effect on the distribution of metal‐bound OC at regional scales.